Fivefold twinned silicon crystals grown in an A1–16 wt.% Si melt

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Abstract
A crystallographic study of a five-branched primary silicon crystal grown from & hypcroutectic A1-Si melt indicates that such crystals grow from a twinned decahedral nucleus. This nucleus morphology has been observed in experiments on vapour deposition of f.c.c. metals. The decahedron is an assembly of five silicon tetrahedra in twinned orientation, giving a particle containing five twin planes with a common 〈110〉 axis and with a surface composed exclusively of low-energy {111} facets. As the equi-axed particle grows it becomes unstable and each twin plane becomes the centre of a faceted branch growing by the twin plane re-entrant growth (TPRE) mechanism. A model for. the transition from equi-axed to branched growth is proposed, and it is suggested that, in a sodium-modified alloy, the primary silicon particles remain equi-axed because the sodium reduces the growth anisotropy required for the transition.